1. Field of the Invention
The present invention relates to an image forming method, such as an electrophotographic method, an electrostatic printing method or an electrostatic recording method, in which an electrostatic latent image is developed with a magnetic toner. More particularly, the present invention relates to an electrophotographic image forming method, as well as an apparatus for carrying out the method, that is adapted to develop by reversal development a digital latent image expressed by unit pixels which are, in turn, expressed by either an on-off binary representation or a limited number of gradations.
2. Related Art
One type of conventional development apparatus is adapted to develop a latent image formed on the surface of a photosensitive drum (serving as the latent image supporting member) with a magnetic toner of a monocomponent developer. In a typical example of such an apparatus, magnetic toner particles are brought into frictional contact with a sleeve (serving as the developer conveying member), thereby triboelectrically charging the magnetic toner particles to a polarity opposite to that of the charge of the electrostatic image formed on the photosensitive drum. The magnetic toner is very thinly spread on the sleeve, and conveyed to the development region defined by the sleeve and a part of the photosensitive drum. In the development region, while a magnetic field generated by a magnet fixed in position within the sleeve is applied, the magnetic toner is caused to jump onto the photosensitive drum so as to develop the electrostatic latent image on the drum.
In such a development apparatus, it is necessary that a relatively thin, uniform magnetic toner layer be formed on the sleeve. However, such layer formation is readily influenced by environmental conditions, the physical properties of the toner, the condition of the surface of the sleeve, and the like. It is, therefore, difficult to obtain a uniform toner layer. A low-humidity environment, in particular, often results in a non-uniform layer formation.
Other problems arise from the repeated use of the magnetic developer. Since the developer is brought into frictional contact with the sleeve each time an operation cycle (such as a copying cycle) takes place, there is a risk of part of the additive(s) for improving the fluidity of the toner being deposited on the sleeve, or part of the binder resin contained in the developer being formed into a film on the sleeve. As a result, the surface properties of the sleeve may change, the developing ability of the developer may become unstable, or the conveyance of the developer to the electrostatic latent image surface may become unsatisfactory.
In order to assure high quality of copied images, various efforts have recently been made to reduce the size of toner particles in the toner layer. For example, the use of a particle size of from 4.5 to 8 .mu.m makes it relatively easy to assure increases in the resolution and sharpness as well as the faithful reproduction of an electrostatic latent image, so that the printing density of an electrophotographic laser beam printer is increased from the conventional level of about 300 dpi to the level of about 600 dpi. Toners having such a small particle size, however, have the following disadvantages when compared with conventional toners: the amount of charge per unit volume increases and, in addition, the amount of the fine particles having a particle size of 5 .mu.m or less increases. Accordingly, a toner with such a small particle size has an increased area for contact with the development sleeve, thereby causing the surface of the development sleeve to be easily contaminated, which raises the risk of a ghost image being formed or the image density being lowered.
Other disadvantages of toner having such a small particle size arise from the fact that the toner has a larger surface area than a more conventional toner, and that the proportion of the magnetic component is increased in order to prevent scattering of the toner. Since the magnetic component contacts the surface of the sleeve more often than usual, if the sleeve has a coated surface, the coating of the sleeve is easily scratched. Further, a relatively large amount of fine particles contained in the toner having a small particle size is, due to the mirror image force of the fine particles themselves, strongly electrostatically confined by the sleeve. Thus, fine particles tend to deposit on the sleeve in the lowermost layer of the toner, thereby preventing the other portion of the toner from being sufficiently charged by friction with the sleeve, thereby lowering the developability of the developer. Since this phenomenon is conspicuous in the non-consumed portions of the toner, the portion of the developer containing such toner portion exhibits a different level of developability from that of a developer portion where much of the toner has been consumed. Thus, such a phenomenon results in a sleeve ghost being formed on the developed image.
In order to prevent the above-described phenomenon, and to stably form a layer of uniformly charged toner, it is necessary to use a contact regulating member, such as an elastic rubber blade, disposed in contact with the sleeve, so as to prevent deposition of a fine particle layer.
There have recently been demands for increased printing speed. However, higher printing speed has increased the risk of damaging the coated surface of the sleeve. That is, the risk that the coating may be easily deteriorated, peeled, or scratched is enhanced.